1. Field of the Invention
The present invention relates to a semiconductor device having a plurality of thin film transistors on a substrate that use a semiconductor layer annealed with a pulsed laser as a channel region, and more particularly to a technique for overcoming variation in characteristics of the thin film transistors.
2. Description of the Related Art
Organic electroluminescence (hereinafter referred to as xe2x80x9cELxe2x80x9d) display panels using organic EL elements have been known as flat panel displays. While liquid crystal displays (LCDs) are widely used as flat panel displays, LCDs merely transmit or reflect light from a light source separately provided, and are not self-emissive. On the other hand, organic EL display elements are used in a self-emissive display for supplying a current to an organic EL layer provided between an anode and a cathode to cause the layer to emit light, and therefore back lighting is not required, as opposed to the LCDs. Organic EL display devices are expected to become the next mainstream flat display panel for that reason and because they are thin, compact, bright, and low power consumption devices. Particularly, it is expected that an active matrix organic EL displays including a switching element for each pixel will become mainstream devices among next-generation flat panel display devices because each pixel thereof can be independently turned on and the amount of current can be decreased, making it possible to maintain a high display quality when used for high resolution large screens.
In such an active matrix organic EL display, a thin film transistor provided for switching for each of the organic EL elements (light emissive elements) arranged in a matrix individually controls supply of current from a power source to the organic EL element, and causes the element to emit light at a luminance in accordance with a data signal.
Known thin film transistors used as a switching element for each pixel include a semiconductor layer of amorphous silicon, polysilicon, or the like, which forms an active layer. Although for simplicity in manufacturing amorphous silicon has been used is for the active layer of the thin film transistor, polycrystalline silicon has come to be used in order to enhance operation speed and implement a display device with a high resolution. To form a polysilicon layer on a substrate having a low melting point, a laser annealing process is first performed on a silicon film formed in the amorphous state, thereby polycrystallizing the amorphous silicon.
Such a laser annealing process is usually performed by sequentially scanning the irradiated object with a pulsed laser beam (sheet beam) shaped such that the irradiated area will be an elongated rectangle. However, as the laser irradiating conditions are varied and not always the same, resulting variation in crystal properties of the thin film transistors leads to a difference in properties among the thin film transistors located at different positions on the same substrate, resulting in variations in pixel luminance.
The present invention has been conceived in view of the above-described problems, and aims to provide a semiconductor device capable of avoiding problems in thin film transistors caused by laser annealing.
The present invention provides a semiconductor device having on a substrate a plurality of thin film transistors using a semiconductor layer annealed with a pulsed laser as a channel region, wherein said semiconductor device includes a pair of thin film transistors connected in parallel for supplying a driving current from a power source line to a driven element, and channels of said pair of thin film transistors connected in parallel are displaced from each other in a direction parallel to a scanning direction of said pulsed laser.
By thus disposing a pair of thin film transistors for supplying the driving current to the driven element such that they are displaced from each other with respect to the scanning direction of the laser, effects of variations in energy in a single shot and between shots on the characteristics of the transistors can be diminished, thereby reducing variation in luminance of light emitted from organic EL elements forming the respective pixels of, for example, an organic EL display device.
The present invention also provides a semiconductor device having on a substrate a plurality of thin film transistors using a semiconductor layer annealed by scanning with a pulsed laser as a channel region, wherein said semiconductor device includes a pair of thin film transistors connected in parallel for supplying a driving current from a power source line to a driven element, and channels of said pair of thin film transistors connected in parallel are spaced apart along a direction parallel to a scanning direction of said pulsed laser by a distance set longer than a movement pitch of said pulsed laser in said scanning direction.
As the line beam moves by a predetermined pitch between shots during laser annealing, a single region is irradiated by a plurality of laser beams. The variation of laser output energy among the plurality of shots causes a difference in quality of the polycrystallized film after the laser irradiation (annealing) process. When the channels of the thin film transistors are spaced apart along the direction parallel to the scanning direction of a pulsed laser by the distance longer than the pitch of movement of the laser, the two thin film transistors can be prevented from being annealed by the same laser shot, thereby greatly reducing the chance of simultaneously causing a problem in the two thin film transistors.
The channels of said pair of thin film transistors are preferably disposed so as to align as a substantially straight line along the scanning direction of said pulsed laser.
The above arrangement can achieve separation of the two thin film transistors while diminishing adverse effects on arrangement of the remaining elements.
The channels of said pair of thin film transistors are preferably disposed on opposing sides of said power source line, thereby achieving an effective arrangement of the pair of thin film transistors.
According to one aspect, the present invention provides a semiconductor device having on a substrate a plurality of thin film transistors using a semiconductor layer annealed with a line pulsed laser as a channel region, comprising at least one element driving thin film transistor for supplying a driving current from a power source line to a corresponding driven element, and a switching thin film transistor for controlling said element driving thin film transistor based on a data signal supplied when selected, wherein said element driving thin film transistor is disposed such that a longitudinal direction of said line pulsed laser traverses said element driving thin film transistor in a channel width direction thereof.
According to another aspect of the invention, the present invention provides a semiconductor device having on a substrate a plurality of thin film transistors using a semiconductor layer annealed with a pulsed laser as a channel region, comprising at least one element driving thin film transistor for supplying a driving current from a power source line to a corresponding driven element, and a switching thin film transistor for controlling said element driving thin film transistor based on a data signal supplied when selected, wherein said element driving thin film transistor is disposed such that a channel length direction thereof runs substantially parallel to a scanning direction of said pulsed laser.
In laser annealing, variations in laser output energy are observed within a single irradiation area of a pulsed laser and between laser shots. Meanwhile, an element driving thin film transistor used in, for example, a semiconductor device such as an active matrix display device serves to drive an EL element. When polycrystalline silicon is used for an active layer of this transistor and this polycrystalline silicon TFT has such a driving capability as, for example, the mobility of 100 cm2/vsec,the element driving TFT is often designed to have an extremely long channel length with respect to a channel width thereof. Therefore, by disposing the element driving thin film transistor such that the channel length direction runs substantially parallel to the scanning direction of the laser or that the longitudinal direction of the laser traverses the channel width direction, annealing of the entire channel region of a single element driving thin film transistor by a single shot can be prevented. Such an arrangement can be achieved by providing the element driving thin film transistor with a channel length longer than a pitch of a single movement of a pulsed laser. Consequently, when a plurality of driven elements are formed on a single substrate and a plurality of element driving thin film transistors are formed corresponding to a plurality of the driven elements for supplying a current thereto, each of the thin film transistors is annealed through a plurality of laser shots, so that variation in energy among the shots is uniformly put on each transistor, thereby averaging the characteristics of the respective thin film transistors. As a result, variation in luminance of light emitted from organic EL elements forming the respective pixels of, for example, an organic EL display device can be significantly reduced.
According to a further aspect of the present invention, in the above semiconductor device, a channel length direction of said element driving thin film transistor does not coincide with a channel length direction of said switching thin film transistor.
According to a further aspect of the present invention, in a semiconductor device as described above, a channel length direction of said element driving thin film transistor coincides with a direction in which a data line for supplying said data signal to said switching thin film transistor extends.
The switching thin film transistor is disposed near an intersection between a selection line for selecting the transistor and a data line for supplying a data signal, and is often arranged such that the channel length direction of the switching thin film transistor is substantially parallel to the direction in which the selection line extends. When, in such a layout, the channel length of the element driving thin film transistor is disposed to extend in a direction other than that of the switching thin film transistor or in a direction the data line extends, lengthening of the channel length of the element driving thin film transistor is simplified, thereby facilitating arrangement of a plurality of element driving thin film transistors connected in parallel or series in a pixel. As a result, the element driving thin film transistors having balanced characteristics can be implemented due to reduced variation in laser annealing conditions among the element driving thin film transistors.
For each of the above semiconductor devices, said driven element is an organic electroluminescence element, and said semiconductor device is an organic electroluminescence display including the organic EL elements arranged in a matrix.